Integrated lignocellulose digestion and recovery process



G. slvoLA 2,730,445

INTEGRATED LIGNOCELLULOSE DIGESTION AND RECOVERY PROCESS Jan. l0, 1956 4 Sheets-Sheet 1 Filed March 21, 1952 INVENToR.

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Jan. 10, 1956 G. slvoLA 2,730,445

INTEGRATED I..IGNOCELLULOSE DIGESTION AND RECOVERY PROCESS Filed March 21, 1952 4 Sheets-Sheet 2 INTEGRATED LIGNOCELLULOSE DIGESTION AND RECOVERY PROCESS Filed March 2l, 1952 G. SlVOLA Jan. 10, 1956 4 Sheets-Sheet 3 INTEGRATED LIGNOCELLULOSE DIGESTION AND RECOVERY PROCESS Filed March 21, 1952 G. SlVOLA Jan. 1o, 1956 4 Sheets-Shee'fl 4 IN V EN TOR. 65e-aia? ,raz

Q 4 7' TR/Y'y United INTEGR;\TE`D'v MGNOCLLULQSE lIoEsN AND RECOVERY PROCESS Georgesivalasaui; ErminiaV Application Marchv 21-,v 1.952', Serial No. 257,818 11 Clarins: (Ci.- 92111) The present inventionrelates to the art of'producing' pulp from cellulose fibrous materials, and,more`particu larly to an improved multi-stage acid-alkali cooking process using a low acid/wood' (fibrous-materiallratio of the order of about 3:1 for producing pulp frombrous nl'ate` rials containing cellulose and for ther recovery of heat and chemicals of the residual-liquors.-

It is well known to thoseskillediirr the art that'in the ordinary suliite process using calcium'bisulte-witli-anv excess of sulfurous acid in tli`e` cookingfliquora'n enormous amount` of work hasbeen" spent on the'development of a method for a fullV andeconomical'recovery'of thev residual liquor, the so-called' waste-liquor, out no satis-` factory solutionv to theproblern has as yetbeen found The common practice, therefore; has been to-dispose the residual liquor into streams or bodies'of water. Asis well known, this practiceihas resulted in-seriouspollutionof the water. The mainobject ofthese'priorfattemptsfor recovery, apart fromthe-preventionlo'f. pollution-of the streams and bodies ofwate'nfhas been connect'etlwvithI the possibility of utilizingtheheat*value oftheorganic" compounds in the liquonwhereas'fthecalcium*contententr the liquor possessedionly a*vvery"small-economic value. it is also known that`tl1e use 'o sodiumfinsteadof cal-V cium in the cooking liquor, such as sodiumsuliteV or' sodium bisulte, gives suchl advantages that 'theiresultir'l-g" pulp is of a bettervquality-than" that obtained bytlrecorr-' ventional calcium bisulte processi Suchhprocesseshve been used in spite vof the facty that the price'forsodiuinis much higher than the price for calcium:L Inrviewot this dificrence in price," very serious eiortsfhave" been made to developa satisfactory recovery-'s'ystemffor the'- re-use of the sodium compounds-andthe utilization o't'" the organic compounds ior'f'theidevelop'mentoffheatgjetc;

Such a prior process is described k-n'ftheSwedish Patentz compounds from residualliquor Vresults'f'rom the-*fact ythat in burning the liquors concentrated by evat'aorationf'noty only sodium carbonate Vwasr formed but' also considerable" amounts of sodium'sulide. The complete-convers'ioriot such sodium sulde vinto sodiumbicarbon'ateand hydro gen' sulfide involved a` very diilic'ult task tas* suine unchanged sodium'suliidererriained;v lf the"recov'eredand` converted sodiurncompounds were usedfoi' tlieimakingof sodium suliite'or sodium-bisulfe, some other-coin# pounds were also formedsucli'as sodium'v'thiosulfate-'and sodium polysultides which causedgreatdifculties ini'sub# sequent cooks ev'en-l though the' amounts'4` of"thes'e'salfs" were comparatively small. l These diiict'llties'were due mainly to the fact that'sodiun sulfide', sodium'thiosulfate' and sodium` polysuliides-reactedrwitliv sulfur dioxideY to7 form free sulfur.` npracticektheebad experience'sfhave caused thosefskilledffin the art Vtosuggest-stliat'a pulp mill using the 'sodium sulite---or` sodiumbisulfiteecooling tes Patent" KO Patented Jan.' l0, 1956 process should be operated'in connection-'with'a sulfate pulp mill. In such a case, the unconverted sodium'sulfide from the"sultern'ill'recovery system can-'beused in the sulfate cooking-liquor @increase` the suldity ofthe sulfate cooking-liquor.

It has been discovered'*th-at"tlre` foregoing difficulties' arising from' the' conversion of recovered 'sodium sulfideA prsent three-stage is overcome by' th'e utilization of-the cooking process which uses in the rst acidstage'so'diumv suliite or sodium` bisulte'made'fiomthe fully converted partof` the sodium carbonate'and'soiiiuxrr'suliide. Such fully converted part canfbe` separated-aspure crystals o'f sodium bicarbonatevfroin'tlesmelt solution which' is treated in one continuouseoperationin: atoiver'witha excess of nearly pure-and lcompressed carboidioxide' gas. The resulting conversion"of"the"sodi1rnsulide canl be' completed or only' partly'cornpletedl'v When'it'is only partly completed; 'the'solution' containingv the sodium sulfide besides the sodium carbonate and'sb'diur'iiI bicarbonate is used for the injection inAvthe Ase'con'dstageof my threestage process" to; converttheT acid^cooking`isolution'into an alkaline cooking'solution'. Inth'issecoiidistage a'svvellk as in the third/stage ofthecooking processi, 'the presence of sodium sulde doesnotl caseany diiliculties'or disadvantagesv as the 'cooking'conditions are' alkaline.'

It likewise has been-foundthat in" tlie` stage cooking process;-alkali'usedi'for tli'emaking ot' tlie sodium suliite or the sodumbi'sulte' 'canf"'co"r1taiiun`con verted sodium sulfide fronr'th'esmeltsolutioiiand: there# forc even' thel mother liquor fronrtlh'e"sepr'ationt'of the sodium bicarbonatefcrystalsicanbe used'for th"e"rnakin`g of' acid cooking 'liquor fori-the firststage"because` in 'the' following stages alkaliis"injevc'ted or present 'in 'excess and no diculties'canirestilt from'thepres'ence of such'uncon'- verted sodium sulfide:

It has also:been-found'ftlrat inthe presentv improved" process Athe iiistsfageof'- cookingffis conducted vwith a" sodium bisultite withfanf'ekce'ss o'f-'sulfurou's acid 'and the second stage involvesiai alkali'injection of-'a 'carbonate 'ot sodium fori convertingdhe cokngliqu'or fro'm neutral or acid to alkaline'withoutseparating theliquor'from'itlie pulp and thethirdstagfo thefc'ookingfptocess"is'coiil ducted with the alkali-icookingrliquorcontainingmainlyv a carbonate ofy sodium fur compounds" derived` from' the pev'ious operations of the cooking'wherebyfthepulp-'producedisfseparated from the residual liquor and the liquorfis concentratedand burnt in a4 recovery l unit Jwitii* a4A furnace" and la' boiler to It is also anobject-of tlieipresentf inve'tin' tofconf vert sodium-carbonate -occurringlinthe' solution obtained from the smelt-into-sodiumibicarbonate'by meafnsofcarbon dioxidev whileat the saine" time' also tieni/'erting'` so'l dium suliide in'th ysaid''solixtiontiritosodiuin bicarbonate and'gaseoushydrogncsuliideA by m''ans ofthe carbon' dioxide. T he' present invetion likewise contemplates lthe neutralization 'ofY Ysodium bisultegfree"sulfurous 'acid 'arid any acid'vforrned during the acidicokingstage' in Ione and the samefdiges'tner'-in-which tliefcdokingis :coductedand which neutralization" changes thefple'lrto' 'a 'value between present' threei 3 7 and 13 whereby substantial economical advantages are gained because the machinery and equipment used for separation, concentration and burning of the residual liquor and solution of the smelt can be made of the same material as used in the conventional soda and sulfate process.

It is likewise within the contemplation of the present invention to recover the carbon dioxide liberated from the digester during the neutralization and the cooking so that it can be used for conversion of sodium carbonates into sodium bicarbonate and sodium sulfide into sodium bicarbonate and hydrogen sulfide, all of which can be reused in the process.

A further object of the present invention is to re-use the sulfur compounds needed in the cooking which compounds are recovered in two ways, (l) by the recovery of all the free gaseous sulfur dioxide remaining available after the first acid stage of the three-stage cooking process and the use of these gases for the making of the cooking acid and (2) by the recovery of the hydrogen sulfide evolved in the conversion of the sodium sulde of the smelt solution and the burning of such hydrogen sulfide to sulfur dioxide which also is likewise used for the preparation of cooking acid.

Another object of the present invention is to recover and re-use all the sodium salts used in the novel threestage cooking process, to wit: mainly for the preparation of the cooking acid containing sodium sulfite or sodium bisulfite needed for the first acid cooking stage and for injection of alkali for the neutralization of the acid cooking liquor involved in the second stage and for the provision of the necessary alkali for the alkaline liquor in the third stage of the cooking process.

A still further object of the present invention is to recover and re-use all the carbon dioxide needed for the conversion of the sodium carbonate and sodium sulfide of the smelt solution and such carbon dioxide is recovered firstly (that is, primarily, principally, or mainly) from the digester when liberated during the neutralization stage and alkali cooking stage, secondly when liberated in the acid making operations where sulfur dioxide reacts with sodium carbonate or sodium bicarbonate, thirdly in other auxiliary operations, such as the conversion of sodium bicarbonate into sodium carbonate by heating, and fourthly from the flue gases containing carbon dioxide.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawings, in which:

Fig. l is a flow sheet of a plant capable of carrying the present invention into practice for the preparation of the cooking acid used in the first of the novel multi-stage cooking process;

Fig. 2 represents the equipment used in carrying the novel three-stage cooking process embodying the present invention into practice;

Fig. 3 illustrates a flow sheet of the novel combination of operations involved in the treatment of the residual liquors, to wit: their evaporation, burning and dissolution of the smelt as well as auxiliary operations; and

Fig. 4 illustrates a ow sheet depicting the conversion operations involved in the treatment of the smelt solution with carbon dioxide.

Broadly stated, the present invention provides a novel combination of cyclic self-supporting operations for the recovery of chemicals and heat and for the novel threestage pulp cooking process, in which the first stage is conducted with an acid cooking liquor, the second stage involves an injection whereby the acid liquor is converted into alkaline liquor, and the third stage is conducted with an alkali liquor having a pH from 7 to i3 measured at room temperature.

The present invention may be carried into practice in any appropriate equipment, as those skilled in the art will readily understand. However, it is preferred to use the equipment illustrated in the accompanying drawings.

In Fig. 1 the reference character 10 designates a gas fan for the mixture of hydrogen sulfide and carbon dioxide gases coming from conversion tower 104 (see Fig. 4) and blowing said gases through a safety device 11 into a combined hydrogen sulfide and sulfur oven or furnace and steam boiler 15. Air blower 12 blows air through a safety device 13 to the burner of the oven or furnace 15 in which burner the air is mixed and burned with the hydrogen sulfide gas as well as with liquid sulfur from melting pot 14. The heat developed by the combustion is used to generate steam which flows through pipe 16 to be used in the process. The gases resulting from the combustion pass through gas cooler 17 into sodium sultite tower 18 where the SO2 contained in the gases reacts with a solution of sodium sulfite to form sodium bisulte or sodium bisulte with an excess of sulfurous acid in case such an excess is desired. ln the summer, it is dillicult to make an acid with such an excess, and, therefore, the absorption in the tower 18 will be conducted only to the stage of a sodium bisulte solution. An attempt to make a solution with an excess of SO2 when the water is warm would lead to the result that sulfur dioxide would pass out into the air together with the inert gases going out from the top of the tower. When a solution containing only sodium bisulfite is made from the sodium sulfite solution pumped into the tower 1S, the use of this solution differs somewhat from the use of a solution which also contains free SO2 in excess. ln Fig. l, both cases are shown and will be fully explained hereinafter. It was stated hereinabove that a solution of sodium sulfite is pumped to the tower 18 and in both cases this solution cornes from the tank 19 and is pumped to the top of the tower 18. From the bottom of the tower 18 the acid in both cases passes to a storage tank 20. When an acid containing sodium bisulfite without an excess of SO2 is made, only that part of the acid which is needed to charge the digesters passes pump 21 into the low pressure acid storage tank 22 to be strengthened further in a manner similar to the operation when an acid with an excess of S02 comes from the tower 18. The other part of the sodium bisullite solution is circulated through pump and line 21a into the neutralization tank 97 where it neutralizes a solution of sodium bicarbonate to give sodium sulfite as explained in connection with the operation of the tank 97 hereinafter. When an acid containing an excess of SO2 is made in tower 13, all the acid is pumped from the storage tank 20 by pump 2l to the low pressure acid tank 22. From the tank 22 any acid which has reached it according to both the alternatives mentioned can be transferred by means of the pump 24 to the strong acid tank 25. The acid is pumped from storage tank 25 with pump 26 through line 36 into the bottom of digester 43 (see Fig. 2).

The material to be cooked has been filled into digester 43 and this filling operation is represented by line 45. A modern digester is usually provided with a circulating system with a heat exchanger. The cooking is, for instance, done by means of equipment known to those skilled in the art, by having the cooking liquid circulated through heat exchanger 44 whereby heat is supplied by steam from line 7S. In this case, the cooking follows the method embodying the novel three-stage cooking method in which the first stage is the acid stage. During the first part of the acid cooking, the excess SO2 is relieved through line 46 through cooler 48 and further through line 49 to injector 30 into which pump 27 circulates acid from tank 2S in order to develop suction and provide better absorption of the SO2.

After completing the acid stage of the cooking, SO2 is relieved from the digester so that the free SO2 content of the digester becomes as low as possible (1% SO2 or less) and the released gas goes through line 47 (see Fig. 2), cooler 50 and pipe 51 into low pressure acid tank 22 (see Fig. l). SO2 and other gases not absorbed in tank 22 go from this tank through pipe line 2S into the bottom of the sodium sulfite tower 18 where the SO2 is absorbed, and the inert gases leave at the top of the descrlption likewise apphes'fto the operatiorrwhe'rr the* free SO2 is recovered Afrom digesters'f 'a vrr'ianrier typical -for the conventional-'sulfite method.-`

Greatv advantages are secured 'by'usi'n'g' a system of liquefying the recovered SO2 sorthat liquidSOr yis available for operations which'are `c le'sorlibelhereinafter. The tower-'acid' coming fromftower 18 (see`j-Fig`.'"`-1)- passes' through feedtankv andv is p'rnp'ed-iuto acid `storage tank- 22. Pump 23 transfers*this; acidI'through"line v"36." to digester 43'(see Fig-2) to vsupply th'eamount of liquid" needed for the acid stage ofthercookingf'othe'ftibrous raw Ymaterial. Cooking "begin' by having theacid-l cir-` culated through heatexchang tank-34 (see'Fig.-- l) through-the measuring-'tank`33'tsee Fig.'V 1) and pipe line 36 into the digester. The moment of such injection ofliquid SO tmight'be'-chosen*atwill.

For instance, it can be applied virn'nediately after the acid has been pumped to the digester,` in Whichl'caseno 'timeL has been allowed for rthe penetration'of the'sodiumbisu'l lite into the material to be'cooked or it might take place later, as for instance up to'4 hours later', in order'to allowv time for penetration ofthe sodiumbisulfite into the chips'.

At'the beginning of thecooking` tlegases; mainly inert' gases, are relieved through 'line"47-a1:td' cooler 50 v andy Afterthe pressure pipe 51 into'tower 18 (seeFig. v1). in'the digesterhas increased,- forf instancajup to? 5' at-` mo'spheres, gauge' pressure; or'hi'gher,- the SO2 gas in the digester is'relieved'throughline '49 '(see' Fig; 2); cooler 48 through line 38 (see Fig. 1)' intoa liquefactionplant* 35 which can be arranged 'according' to,Yforminstance,v

Norwegian Patent No. 73,832l (Somer). Usingv the Tmetlil" od outlined in said'` patent on1y-abo`ut` 70% of -the 'SO2 coming` from the digester can be liqueiied'and this lique fied SO2 goes into storage tank- 34. The'partoftlie'SO'zf` which leaves the liquefactionrplaiitY -35-a'sfa VgasiscOnf ducted through pipe 32 intothe' highpressure'acidtanlcE where the pressure 'is'atleastfaboutratiriospheres;v gauge pressure. In or'der' to'get2 more SO' liquefied', the* pressure' of thev free gaseous SOzin the uppery part' v'of th'e"v acid tank'fZS is raised by relievingapart of the conc'eii trated SO2 from the digester under pressure thro'gl'fthe into the acid `tank 25. The'concentrate'diSOgafs" from pressor 31 to' the' same gas -pressure'as' thegascoilniiig Regardless of whether'th'e" conventionalsyst'em of're'# lief has been used or' the system 'u'sin'g" liquid ASO2 de scribed herein has" bee'rra'pplied,v theVTV further" operations involved in the multi-stage' cooking take place ias'follows':

In order to keep the amount of liquid eventually'to be evaporated as low'ias possible, some a'cidcc'xkinglliquidV is drawn from the digester' at the 4fend 'olf-the' -ac'idl cooking stage through the 'pipe '41 (see"Fi'g`.'-2)' into'the 'tankl 89' (see Fig. l).

From the tank 122 '(s'ee Figy4), a solution offsodium salts' containing mainly' sodiur'rr-carbonate 'and Asodium bicarbonate is pumped'throughlpipe t0-into the pressure'4V side of the pipe of the circulating purnpof 'th`e'1"di'g'e's"ter.` When' this injection of alkali isca'rri'e'd`out,`ca'rbn through line 52 y(see Fig; 2) wliicliconducts" the ga'scon# taining mainly CO2 and also sorn'e'SOgto a scrub'l'ierSSy where SO2 is removed from the gas mixture bya circulating liquid containing sodiumfbicarb'onate This" liquid' is circulated inthe scrubber53until" sufficiently" 'usm rap the'acid -rnakiiig by I7p''u'nipir'ig-'fi't'ftothe l"sdiiiri sultiteitahll n Lfand-steam is"supp'lied v through line -75 to thefheatercha'nger 44. Afterthis' operation, aninjection of`liquidSO2 is effected' from*y roXider` is liberated in the digester and'tlie"digesterisrelieved the seribb xsvsupiplie'd V'tothe scrubber from'sodium 'bicarbonate tank 116 (see thereis practicallyfhdsodiurnfsuliidefin:'the Vsolution used for the alkaliinjee nintothef digester Therefore, the alkali'co'okingliqi onta s"'"'mainly`sodium carbonate, somey 1 sodiunrbicarbonate and'v-so'diur'nYV sulte apart from the 'salts' formed' of' organicsubstances'` in the" digester as well asisuch orgz'inic'l substaeswhiclr' do lnot combine witl'alkali.- Orrtheot han inthefcas'e that the conversion of'the smelt as not'fbeen'vcarried out to completion, there: might'ral'so' 'be present in vthe injection liquor some-'sodiumfsulde which'rinthe' digester reacts with free SO2 forming sodium l'thiosult'a-te"and under certain conditions also 'sodit'u'npolysilfide's.'- However, as the cookingnow is'carried out'wit'h 'alkali,'there is no danger of format'i'rr'of-` free' sulfur-lin thecooking liquor and the sodiuinthiosulfat'efor Asodiiirirfpolysullides can dono harm to the cooking opeatiris l Aftertheicompletionof thealkali stage of thecooking, thecontet' of'jthedige'st'eris" blowriintov the blow tank 56"`(seeV-Fig. 2)A from"'whe'r"e"the'fmikture 4of pulp and liquor ispunipdthrogh"pipe and regulator 61 to the'pulp 'washers 62 where theire'sidial'liquor" is Vselgiarated from'tlie' pulp'aidthe-stro-ng liquorrecovered goes to Apartof this strong sepa-- ratedliquor'fis pumped through line 68into the blow tank Sti to`cor1'vey the' pulpr'fro'mtheiblow tank tothe pulp liquor being pumped from the evaporators to a storageA tar'ikffis Afrom"abou't"b5()`ft'ce' about 55%.V Fronithe tank 7tfthe tconcentrated" liquor cascade evaporator 76,y Wherelthe u gases coming fronr'arecovery boiler 71 through duct evaporate the concentrated yliquid from" storage'*tank""70lfurtlieri so lthat the`d'ry'subs`tance content of-ftheliquorfgoing ito'a Af eei"tlarikv 72'isffrom about 60 to about %.y Into feed'i'tank72, salt cake orsome other cheap"sdiiim-sulvfur'compound' is added' to cover'the' alkalilos'sesduring' th'ecycli'c operation; From feed tank 72, 'the''strongiliquor goes'tou recovery'furnace71 where it isi-*burnt underireducing conditions to form a smelt containingrnainlysodium" carbonate andv sodium sulfide and-runs: out from tliefur'nace'to a smeltI dissolving tank SL Whiley burning', theorgianic matter' developsl heat in furnacet7'1 which s'u'sed to'generate lsteam and this steam gees first toithetstearrr'turb'cgerirator73 having several bleeds for differenty pressuresof stearn'to be used in` the' pro'cess?"suchas.. cooking' pipe" line75`(see Figs, 2 and V3). The arn'ou'n'tl of'ys'teamdeveloped isso'large'that it' is'v sutil- 2 cient'l forall thepower"'andt steam requirements of the' improved processir` Y The residual liquor is boiled in the evaporators 69 and steam is formed from the`watei of -theresidual liquor. Afterpassirigthrough the' evaporator vunits',`the steam is 'condensed and it' is called" secondary condensate. The

condensate has a'slightbut not vunpleasant odor and it is "su-Iii'ltes'olutions to be' Sodiumibica'rbonate tied as, for instance, boiler feed water. Therefore, it must be purified before being used. The purification of the condensate is not a difficult task. It can be done as follows: 30 to 60 grams/m.3 NaOH and 20 to 30 grams/m.3 alum is added to the condensate and then mixed, l to 3 hour reaction time is allowed and then it is filtered through a sand filter. The condensate can be purified by filtering it through a charcoal filter. Activated silica gel is also a good filtering medium. By treating the condensate this way it can be used as shower water on the drying machine, particularly when making highgrade pulps because the condensate is practically mineral free. Of course, it can also be used in bleaching operations where pure mineral free water is needed.

The fiue gases going through cascade evaporator 76 pass through pipe into an electric filter or precipitator 77, where the sodium salts and dust carried by the iiue gases are precipitated and conveyed to the feed tank 72 to be rc-used. The main part of the ue gases continue to stack 78 and a part of the flue gases containing CO2 is blown a by fan 79 through pipe 80 into scrubber 124 (see Fig. 4).

In dissolving tank 81 (see Fig. 3), the smelt is dissolved in alkali wash water coming from the wash tank 82 or the dissolving water might be alkali wash water from the washing of conversion tower 104 (see Fig. 4), from which it comes to dissolving tank 81 or wash tank 82 through pipe SS, or fresh water might be used. The solution in the dissolving tank is made so that it contains about 150 grams Na2O per liter, which is the highest content of alkali which can be safely used. The smelt solution is pumped from tank 81 to mixing tank 83 into which magnesium salts may be added in a known way and in which mixing is carried out. From tank 83, the smelt solution is run to clarifier 84 where the dissolved smelt is clarified and the calcium, iron and silicon compounds derived from the wood during the cooking are removed by the sludge going out through the bottom of the clarifier and pumped to washer-clarifier 82 where the sludge is washed free of sodium compounds and from where the wash water goes into dissolving tank 81. Wash water used to wash the sludge from the smelt can be the same water that is used to remove sodium bicarbonate crusts from the conversion tower 104 (see Fig. 4). The sludge from the bottom of washing tank 82 is disposed of to the sewer.

The clarified liquor from clarifier 84 is pumped through a heat exchanger 85 where it is cooled down to about 35 to about 40 C., after which it goes to a storage tank 86.

As described hereinbefore, at the end of the acid cookso that steam and SO2 are removed as ash and are conducted through line 29, for instance, to acid tank 22 to heat up the acid and to absorb SO2. The liquor from the bottom of the expansion vessel 90 goes into neutralization vessel 94 in which the sodium bicarbonate used for the neutralization comes either from the sodium bicarbonate tank 116 (see Fig. 4) by means of pump and line 119 or from the sodium bicarbonate tank 111 by means of pipe 110 and pump feed tank 114. The filtrate from the sodium bicarbonate filter 112 goes into tank 114 via tank 113. From feed tank 114, the sodium bicarbonate solution is pumped through line 117 to the neutralization tank 94 (see Fig. l).

Carbon dioxide evolved in tank 94 (see Fig. l) is let out through pipe 96 and passes to scmbber 98 where SO2 is removed from the CO2. Thereafter, the CO2 passes through line 101 and cooler 107 (see Fig. 4) to gasometer 109 orl direct to the compressor 108. The alkali used to remove SO2 from the gas in the scrubber 98 (see Fig. l) comes through the pipe 119 from the bicarbonate tank 116. Enriched wash-solution from the scrubber 98 (see Fig. l) is pumped through line 99 and 100 to sodium sulfite tank 19. The neutralized acid from the neutralization tank 94 goes to the tank 92 from which a part of the solution is recirculated back to the tank 94 to kill foam. There is a branch off from the circulation pump in which line also is a cooler 91 which cools the neutralized acid to between about 30 and about 40 C. Thereafter the neutralized and cooled acid goes through line 102 (see Figs. l and 4) into the trough of the sodium bicarbonate filter 112 to convey the separated sodium bicarbonate crystals into sodium bicarbonate tank 116. The rest of the neutralized acid is pumped through line 95 (see Figs. l and 4) to the alkali for injection tank 122 (see Fig. 4).

For the making of sodium sulfite, a solution of sodium bicarbonate can be taken either from the pump feed tank for filtrate 114 Vthrough pipe 117 to the reaction tank 97 (see Fig. l) or from the dissolving tank 116 for crystallized sodium bicarbonate with the pump and pipe 119, pipe 118 and pipe 117 (see Figs. l and 4) to the reaction tank 97 (see Fig. l) just mentioned. There are no chemical disadvantages involved in the use of this filtrate because it has been proven that the sulfur of sodium thiosulfate in the cooking acid can be as high as 10% of the total sulfur present without causing any trouble when the acid cooking is followed by alkaline cooking. However, there is a great advantage in using the iiltrates to make new cooking acid because in that case the amount of water to be evaporated after the cooking is just as lunch reduced as the volume of filtrate used in the acid plant amounts to. As explained in connection with the description of acid making in the sodium sulfite tower 18, there are two alternatives of operation. Either there was made an acid containing sodium bisulfite or a mixture of sodium bisulte and sodium sulfite or an acid containing sodium bisulfite with an excess of SO2. In the case that an acid containing only sodium bisulfite or a mixture of sodium bisulfite and sodium sulfite is made in the tower 13. a part of this acid is taken from the storage 20 by means of pump and line 21a to the reaction tank 97 to react with the sodium bicarbonate or sodium carbonate. Due to the reaction, carbon dioxide is evolved, and, as the escaping gas contains some SO2, it is conducted from the reaction tank 97 through a pipe to scrubber 98 and then nearly pure carbon dioxide goes through line 101 (see Figs. l and 4) and heat exchanger 107 (see Fig. 4) to the gasometer 109 or to the compressor 108.

The other alternative operation is as follows: When an acid containing sodium bisultite with an excess of SO2 has been made in the tower 18, all the acid has reached the low pressure storage tank 22 and from there possibly also the high pressure storage tank 25. This operation has already been explained. From the acid storage tanks, either storage 22 or 25 (see Fig. l), acid is taken to rcaction tank 97 to react with sodium bicarbonate or sodium carbonate. Due to the reaction, carbon dioxide is evolved whereby the escaping gas contains some SO2 and passes from reaction tank 97 through the pipe to scrubber 98 and then nearly pure carbon dioxide goes through the line 101 (see Figs. l and 4) and heat exchanger' 107 (see Fig. 4) to the gasometer 109 or the compressor 103. From the reaction tank 97, the sodium sulfite solution is pumped into tank 19 (see Fig. l) through pipes 99 and 100.

The clarified and cooled liquor from tank 36 (see Fig. 3) containing alkalinesodium salts, such as i5() grams of Naz() per liter, is pumped through line 87 to the top of conversion tower 104 (see Fig. 4), which is similar to the carbonating towers used in the Solvay soda process and consists of a number of mushrooms and division plates or passettes, one above the other, and the necessary indirect cooling systems. The liquor descends continuously in a well-distributed manner through the tower while it meets a cooled current of nearly pure carbon dioxide gas supplied at a pressure of about 3 kg./cm. and by compressor 108 connected to the bottom of the tower. The carbon dioxide gas comes to the compressor either .agr-:Leones from gasometer 109 `or direct from the 1G02l lsource'sof the process. Iii-the tower, -the 'following.freactionstake place:

ln order to obtain a complete conversion, vanexcess ofCGzfmust be used. Thegasleaving at -ytheftop of fthe tower through the pipe :ste the H25 burner therefore contains notonlyHzS-but'alsoCOa The reactions inthe conversion tower are exothermic and therefore the'temperature in the tower would rise unless controlled by cooling. A temperature of ,-about30 tC. is sought forthe t liquor leaving throughA the draw :opening latthe bottom and upthrough theverticalflpipe for Ythe drawand reaching the discharge point at the funnel which feeds the line 106 to the sodium 'bicarbonatetank .1M-equipped with an agitator. From this tank, the solution containing crystals of sodiurnbicarbonatereaches filter 11.2 where 'the crystals are separated from the mother liquor. After this operation, the crystals lare "flushed with neutralized acid coming through pipe 10250 that they `ow into tank .116 provided with an agitator. iWater vcan also be -usedfor this purpose. through the pipe into a suction tank 113 in which a vacuum is kept by vacuum pump 115. From suction-tank `113, the mother liquor goes into a'pump feed tank114.

In order to Vhave less COzevolved inthe digester, sodium bicarbonate from tank 116, is Apumped ythrough pipe 119 to `the top of sodium lbicarbonate decomposer n120. This decomposer 'is a packeditower provided lwith Ameans .for indirect or direct heating. Whenthe solution trickles down through the tower, heatiissupplied'to the bottom ofthe tower, such as by an'indirect heating element builtin the tower itself, in which the heating is done -by steam coming through pipe 16. -In the vheated solution in the tower, the sodium bicarbonate is decomposed so that carbonvdioxideis evolved.andpasses.throughfpipe 125 and cooler 107 into the gasometer :109 or the CO2 .compressor 108. The=solution containing mainly sodium carbonate leaves via thesbottom of Ythe `decomposerqto a feed vtank 121 from Vwhere a pump yforces it vto. storage tank 122 for sodium carbonateto beused for neutralization inthe digester. In case it -is desired todecompose the filtrate or the mother liquor from the -bicarbonate lter 112, this operation -can-a1so be. carried out in .de-

Ycomposer I120 to -which the .necessarypipe 4linesflead skilled in the art know. Using `anyof the known `meth ods, the flue gases must irst be scrubbed vorfpurilied so that they are free from SO2. lln orderfto do thisvthe llue gases after leaving the electricpreciptator 77v are blown with a fan 79 ,through line 8 0 (see Figs. '3 .and 4) into scrubber 124 (see Fig. 4) in which .sodium kbicarbonate solution ris circulated -by means lof a pump draw ing solution from the bottom' of the scrubber and discharging it into the 'top of the scrubber. Sodium bicarbonate solution is-takenfrorn the sodium bicarbonate tank 116 (seerFig. 4) through-pipe `129. `'lhe-circulatingf.

solution becomes enriched with SO2-fin thefscrubbenand as some soot .whichislcarried overwith the-,f luelgases also might enter into it, itisbest ltopunlpit through pipe 126 (see Figs. 4 and 3) to lthe evaporators 69 (see Fig. 3) where it mixes `with' -the residual iliquor trom'- the digesters The. mother liquor from the lilter Vgoes 1.0 entering-theevaporators rom fthettop iof ftheascrubben the scrubbed-*gas goes-fintopCO2-concentrationplant 123 (see Fig. 4) from wherefit goesV through pipe 1-27into gasometer 109 ordirectly tofcompressorl108.

In pulp mills, as those skilled in the art know, pulp is oftenbleached lwith a wellknown :treatment with caustic soda, such las an -intermediate stage in the Ableaching process, which gives great'advantages'but-which requires caustic soda in such an operation. ilnzthelpresent process, sodium bicarbonateis `available eitherfin form of pure crystals or in formof -a`solution after the conversion of the smelt withcarbon dioxide. i'From such sodiumbicarbonate, itis also possible toobtain ysodium carbonate after the Idecomp'oserwhere all of' thefcarbon dioxide is recovered. This sodium ca'rbonatelcan'beicausticized-with -lime in a wellknown way 'and 'the Isolution Sis strong enough tobe used-in-thelbleaching'process. Itis evident from the description vofthe lpresentrecovery system that Atheultirnate vraw material for-the makingof sodium carbonate consists ofthe salt cake added'to the system via the `feed vtank of vthe furnace. vThis "way of preparing lthecaustic alkali Vfor the bleaching process 'gives an economical advantage, as 'those skilledin the art will readily appreciate.

ln many localities where pulp mills are operated, sodium lcarbonate and/0r causticsoda are available at very lcheapvprices'so vthatitfis morefeconomical to .use them 'in place of salt cake. `lll/'hen it isdesiredto use'these chemicals to cover the alkali'losses of'ithe presentprocess, they can be used either yin the acid fmaking process, where sodium sulte or sodium bisultite is made from car- Abona-tesof sodium, forwhichpurpose .caustic soda also can be applied as an alternative-.or they can be used for the alkali` injection comprising:theineutralization stage of for the production of .pulp Adisclosedzinmy co-pending iapplicatonSerialiNo. 784,322, nowPatent No. 2,701,763. V'-lfhe detail'figures, such as temperatures, .pHs, concentrations, weights and volumes,1etc., as wellas the illustrative examples canibe usedhere'in andare incorporated herein by reference. In my prior application Serial No. 784,322, I have described a three-stage cooking process for producing pulp from ibrous 'materials containing cellulose and-shownl that this Yprocess results in the production of a superior and improved pulp containing libers which have desired chemical characteristics, vsueltas .high alpha contentand whichhave strength characteristics-just as good or better .than .conventional 'sulfate-pulp, vand that this process has self-supportingfcyclic'systems for recoverypof chemicals andiheat. Y

lt-vis .to be observed that the :improvements .over t my Vii1- vention disclosedinfmy co-pending application include the following: v

(1) NazSOa solution is used'to chemically take up all ythe SOzproduced by burning-of. HzS and-sulfur.

u(2) The Ililtra'te lfrom lthe NaHCOs separator is used for acid making .fin place of separated and washed Nal-1G03 crystals from the conversion of fNazCO and.

NazSof the dissolved smeltv solution. y

('3) At the end of 'the acid-cooking stage'some of the acid cookingliquorisfdrawn from thedigester and is used for transportationV `and dissolving of NaHCOa crystals from the'separatororV ilter of the conversion operation.

(4) A better control of the temperature -regulation of'L (6) NaHCOa is decomposedbefore itiis` employed for.

the alkali injection at the end of the acidfgcookingtage,

(7) Secondary condensate, whi`chjis-the evaporated water fromjthe-evaporationof-the-residual liquor, -isus`ed.

(8) SO2, S03 and alkali compounds recovered from the CO2 making from the flue gases are re-used.

(9) NaOH is made from Na2SO4 and silica and other impurities are removed from the system in the same operation.

() Liquid SO2 is made and used in the process.

Those skilled in the art will appreciate that the foregoing improvements are unobvious, particularly in view of the following description:

(l) In the new recovery process ali the residual liquor is evaporated for the recovery of heat and chemicals and therefore it is important for economical reasons to keep the amount of water to be evaporated as low as possible. In other words, the ratio of the cooking liquor to the wood is as small as possible. In the conventional sulfate process, the aforesaid ratio can be as small as 3.2 to l. This is possible because the chemicals used in the process are very soluble whereas in the present process, in which free SO2 has to be taken up by the acid, such a low ratio is impossible to reach, particularly if the gas contains a low percentage of SO2 and if the temperature of the mill-water is high. In my original invention, it is diicult to keep the ratio of acid to wood suiciently small and in practice it is found to be high, such as for instance 4 to l or 4.5 to l. The present improvement provides for a small or low ratio, such as 3.2 to l or lower, if desired. In the present specification and claims, this small or low ratio will be considered as of the order of 3 to 1 or an acid/wood ratio of about 3 to l. Such a small or low ratio can be obtained when the operation is carried out in the following way:

The iiltrate from the bicarbonate lter 112 reacts with the Nal-i803 in the reaction vessel 97 forming NazSOs and evolving CO2. NazSOa is then pumped to the top of the acid tower 18 where it reacts with SO2 coming from the HzS-and-sulphur burner and NaHSOs is formed. Onc-half of the NaHSOa coming from the acid tower goes into the acid tank 22 and the other half goes back to the reaction vessel 97. In this way, the ratio of cooking liquor to wood can be regulated at will. To illustrate this statement, the following two examples are given:

Table for 100 tons of wood I Tons II Tous 1. NMO (45 lig/ton of 1. NazO 4.5

woo 4.5 Y 2. SO: to be combined 2. Combined with S01.- 4.66

with the Na-.O 4. 66 3. Solution containing 3. Solution containing sodium bicarbonate 120.00 sodium bicarbonate. 120.00 4. Acid for neutrali-za- 4. Acid for neutralization: tion: 5% S01 which can 3% S0: which can react 4. 66 react 4. 65 1% SO2 combined.- 0. 94 3% SO; eombined 4. 66

6% Total 802.-" 5.60 6% Total 803..-- 9. 32 5. Total Weight ot such 5. Total weight of acid acid needed 93.2 needed 155,00 6. CO: released in neu- 6. CO2 released in neutralization 6. 4 ation 6. 4 7. Total Weight oi 7. Total weight ot NaQSOs solution NazSOs solution formed 206.8 formed 268.0 8. Weight oi the water 8. Weight of the solution in the NazS Os solufrom the tower contion 195.8 taining NaHSOsm. 277.9 9. Hi8 lor burning 4. 97 9. HiS for burning 4. 97 i0, Weight of S01 formed i0. Weight ot SO2 formed by burning HzS by burning HzS (8% by volume) 9. 32 (8% by vo1urne) 9. 32 11. SO2 which is not 11. Weight of the water needed to iorm 2 in the NaHSOr NaHSO; from the solution 247.5 Na;SO3(9.32-5.60) 3. 72 12. Ratio of Water to i2. Temperature of wawood 2. 48

ter 20 13. Water needed to dissolve 3.72 tons 0I SO2 at 20 C 240. 0 i4. Total quant-ity of Water (2404-1953).. 435.8 Ratio of water to Woo 4.36

- In the foregoing table, a comparison is made between (I), a case when a given quantity of sodium bicarbonate solution is neutralized with strong acid in order to obtain a solution of sodium suliite for the absorption tower 18 in which excess SO2 must be absorbed as free SO2, and (ll), a case when the same given quantity of sodium bicarbonate solution is neutralized with tower acid containing only sodium bisuliite in order to obtain the solution of sodium sulte for the absorption tower 18 in which all of the SO2 from the burner is taken up so that sodium sulte is changed into sodium bisulte.

(1 1) NazO. In practical work the quantity of sodium salts needed has been such that 45 kg. of NazO has been used per ton of wood. Per tons of wood the quantity needed is therefore 4.5 tons.

(1 2) NaaO--SO2=NazSO3. Consequently, according to molecular weights 4.5 tons of Naz() combines with (1 3) The weight of sodium bicarbonate solution according to estimates will be of the order of tons per 100 tons of wood, and this is the given quantity which is assumed the same for both cases.

(1 4) The composition of the strong acid taken for the neutralization is a matter of experience. An acid with 5% free and 1% combined SO2 is very usual inthe sulfite industry.

(1 5 The total weight of such acid needed for neutralization of the sodium carbonate solution containing 4.5 tons of NazO must contain 4.66 tous of SO2 which can react. An acid which contains 5% of such free SO2 is used, and therefore 5 tons of free SO2 are contained in 100 tons of acid and 4.66 tons are contained in (4.66 l00)/5=93.2 tons.

The quantity of CO2 released is (88X4.66)/64=6.4 tons.

(1 7) The total weight of sodium bicarbonate solution (item 1 3) 120 plus the total weight of acid needed for neutralization (item 1 5) 93.2 minus the weight of the CO2 released (item 1 6) 6.4. Result 206.8.

(l-S) The resulting solution of NazSOs contains the 4.5

tons of NazO which were present in the sodium bicarbonate solution plus all the SO2 of the acid (i. e. 5.6 tons) plus the NazO which was combined in the acid (i. e. (62X0.94)/64=0.91 ton), a total of 11.01 tons. 'The total weight of NazSOa solution (206.8) minus the weight of all NasSOs (11.01) gives 195.8 as the weight of water of the NazSOs solution.

(I-9) The quantity of HzS for burning is an estimate,

and the same ligure is used for both cases in the comparison (i. e. 4.97 tons).

(I ll) There is a total of 5.6 tons of SO2 in the resulting solution of NazSOs (see 1 8 explanation above) and when this NazSOs meets burner gas SO2 in the tower, it takes up the same quantity of SO2 when the sodium suliite changes to sodium bisuliite. There is then 2X5.6 tons=l1.2 tons of SO2 in 206.8 tons (item 1 7) plus 5.6 -tons (weight of the SO2 taken up from the burner gas to form bisulte)=2l2.4 tons of bisulfite acid. The percentage of S02 is. then There is now 9.32 -5.60=3.72 tons of SO2 which must be dissolved as free SO2 giving a solution containing ll.2+3.72=l4.92 tons of SO2 in 212.4-{3.72=2l6.l2 tons of solution. This is a total SO2 content of (14.92Xl00)/2l6.l2v=6`.9% and one cannot expect that all this SO2 will go into solution, *wheny the ,burnery gasco'ntav V by volume, lespefcially-lr'1ot in thef'summer.

called combi ed correspo ds to and byv the use of these, onecan :see how'muchtot'al SO2 will be dissolved, at .various temperatures in sollt;

SO2.V In 'thepresent case,.the'bur ner Agascontzllins: only u 8% of SO2 and the partial pressure' of SO2A is therefore' only Water ata temperature of`120WC. A g (L12) 'Ifhe assumption is made thafthewwat'nhzfs :a

temperature of C.

(1 13) In 'the expiananon or itemj'i-ri, itisfod that# the acid can take up` onlygabout""0.8'%rof free S021 The Aweight of water in Y,this acid` was calculatedin item I-S as 195.8 tons and this takes up (195;s 0.s)/1oo- `1.57'

tons of SO2. In item I-ll, y'itwasfoundf'that3l72 tons must be dissolved as free SO2.A There is then tons of SO'2 which will pass out from the vtower unless water is added. The amount of 'waterneeded is'c'a1culated as follows'.

water can dissolve 11.2 grams of SO2 atv 20 C.. and a pressurevof 760 mm. A gas lof 8% `SO2 content gives a partial pressure at the bottom of the tower of ouh/8% of the pressure of a 100% SO2 gas, and therefore according to Hcnrys law only (mamy/100:09 gram of, SO2 will be dissolved in l00jgrarns ofvwater. Consequently, 2.15 tons of SO2 require l p (10o 2.'15)/0.9` 12o" tons of water.

Awas 195.8 tons. The t'otalwater i's'then l95.8-{-240=435.8 tous l (1-15), 435.8.tons per l100 tonszofwoodi-s a ratio'of api'v (rr-5) An acid fwims fons of free S02 weighs 10o tous;

thenn an acid withl- 4.66v tons of free SO2 4weighs' (1oo 4.66)/3=155 tous; Y v. (II-6) The CO2 released computed as in case I-6. (Il-L7) Weight of the solution of sodiumV bicarbonate ..-p 120 plusthe weight of the solution 'of sodium 'bi sulte used for neutralizatiom-; 155

.Y 275.0T minus the CO2 evolved e 6.4`

z'esT.

A table shows that 100grams of (HflQYTASfOr basgllgj (II-ll) 'The sodiurrrbisuliiteV solution'jpfrornfthe tower contains 9.32 tonsof rSO2 from .therburn'er andv just tonlsfin alli This; is combined with sodiumas sodi'ur'n` bisuliitc'. SGzha's fa molecular 'uv/eiglzltdlv whereas sodiurn bisulte has"`a molecular weigh'te-lO/J..

(18.64 1o4)/64=30.4 tous of Nansoz ne@ n n i. e. 277.92-3o,.4 =247.52 tous or wafer. (II-l2) 2517.52l tons ofy water per`l00 ,tous of Wood represent a ratio liquid to Wood ofv approkimat'ely 2.4821.

-'The'jafporesaid examples sho'wwhat the ratio of acid to woodlcan be when the` moisture content of wood is neglected.v In Vthe irst case, itnis 4.3 6 to l whereas in theusecondvcase, the ratio can be as low as `2.48 to l which is lower than any practical commercial or indusltrialratio. When takingvthe ratio 3.2 to 1 versus 4.4 to l,

thereis a dierence in, amount ot 1.2 cubic meters of watfervper ton oigwood `to be evaporated. When, for example, 2.`1` ton s ofubone dry woodis used to make a ton of pulp, ,thenvthereis 2.1- 2.5 cubic meters less of lwaterrto be evaporated or 0.5 to 0.6 ton of steam (35 kg./cm.f", 750 C.) can be saved. v y A (2) lt is stated in my co-pending lapplication on` my original invention that forthe acid making only the separated and washed NaHCOglcrystals can be used. I It has. "f-fbeen foundthat the cooking acid can also bey made from theultrate coming from the hlaI-,ICtJrfilteru 112. yln order toprove Vthis discoyeryto bel'correct, a long series of, cookings were made. Generally, it is believed that NagSzOs causes, all kinds of troubles in the sulite cook-1 4R0-Qing and therefore the cooking acid is recommended to be made of pure yNaI-1G03.nl YWhen making acid of the liltrate,`lythe re mightlbe some NazSOgleftin the filtrateA i coming in the smelt solution and also some unconverted NazS,V which forms with SO2, NazSzOsV (sodium (thio rsulfatteLVor polysuldes. The trial test cooks were made withl an acidnwhich contained so much NazSzO that the sulfur content ofv thelSIazSrz'Os was up to about 20% of they total sulfur contentvof liquor usedl for cooking (NaHSOs-l-SOz-l-NazSzOs). The amount sulfur,f as

ANagSN-Os wasvfound to bethelmaximumwhich did, not

causeany noticeable difference in the cooking operation ouin Vthevquality ofl the pulp. This is due to the fact that the acid cookingpispfollowed vby the alkali cooking. The fact thatdthe iltrate can be usedfor the acid making is an important factor as the amount of water to be evap-v orated will ybe smaller. pWhen making paperv pulp, the amount of water to bey evaporated is 2.5 cubic meters smaller andV when making high alpha pulp, 2.8 cubic meters smaller. This means a saving of steam (35 f kg./cm.2, 750 C.), of about 0.5 to 0.7 ton per ton of pulp.

(3) ItVhs-sl-5een proven` thatrat, the end oi the acid cooking stage, 2 to 3 cubic meters ofvithe cooking liquor.

can beY drawn out of the digestersr43 into vthe tank-39. From sthere, it goes into ythe llash tank 9.0 where it is allowed to liashLandthen it goes into the neutralizer 94 where it is neutralized.` Thereafterpit Acan be used forl transportationand dissolving of NaHCO; crystals comingfrom the filter 112. No separation of lignin takes Y place Vout o f the acid liquid. By doing this, quite a saving ,isuaccornplished as the amount of waterrto be evaporated is ,snraller, dueto utherfact that no extra water'isneed. for ydissolvingof NaHCOs. The saving of steam (31kg/cm?, 750 C.) is about 0.4 to 0.6ton per tim 0..,f. an te. '1 A good functioning of the conversion" tow/ers' (CO2-towers) depends very much on the temperature of CO2 and of the solution of the dissolved smelt. Therefore, it is of advantage to regulate the temperature of CO2 and of the smelt solution. For this purpose, the CO2 being used in the tower is cooled to a selected or desired temperature and the smelt solution in the towers is kept at such a temperature by using heat exchangers built in the towers.

(5) It is also of advantage to use concentrated CO2 and compressed gas as the amount of CO2 dissolved is proportional to the partial pressure of CO2 gas in contact with the solution. In order to accomplish this fact, CO2 and gas before going into the towers are compressed to about`3 kg/cm.2 gauge pressure and then cooled.

(6) It has been proven to be very advantageous to decompose the NaHCOg which is used for the alkali injection before the injection in a separate apparatus. Thus, the time used for the injection is shortened and the liberation of CO2 takes place more evenly as the decomposition of ZNaHCOs to NazCO3+COz+H2O is continuous whereas, if the decomposition takes place in the digester, CO2 is not evolved continuously. This means that the apparatus has to be largerto take care of the batch-like liberation of CO2 and, therefore, the first cost is higher. There is also another advantage which is gained by the decomposition of NaHCOs, namely the digester operation is much easier.

(7) When the residual liquor is evaporated, a large amount of condensate is formed. The condensate is called secondary condensate. This condensate contains certain amounts of impurities and it cannot be used as such, for instance, as boiler feed Water. In the conventional sulfate mill, this condensate is not used, only some heat of the condensate is recovered by using heat exchangers and after that the condensate is sent to a sewer. The condensate contains poisonous sulfur compounds, and of course causes contamination of the nearby waters. ln the present process, the condensate does not contain poisonous sulfur compounds and it can be used to a great advantage after it has been purified with a small amount of chemicals, such as activated silica gel or aluminum sulfate or ltered through ordinary charcoal, etc. Condensate purified in this way can be used as boiler feed water and particularly it is suitable to be used as shower Water on the drying machine when making high grade pulps, because it does not contain any minerals. A considerable amount of heat is likewise saved.

(8) When extra CO2, is needed in the conversion process, is recovered from the flue gas which contains, besides CO2, SO2, S03 and alkali salts, the ilue gas is scrubbed in the scrubbers in which NaHCOa is circulated. SO2 reacts with alkali forming NazSOs, S03 forming NazSOtl, the circulated solution gets dirty and it cannot be used for acid making due to the particles of soot carried with the gas. The circulated solution after becoming fortified with the chemicals is returned to the evaporator plant where it is evaporated and re-burned in the recovery unit. By doing this, the NaI-ICOS used for removing SO2 and S03 can be recovered and re-used and also the alkali and sulfur compound in the flue gases are recovered.

(9) In many instances, it pays to make NaOH for the improved process. This is possible in connection with the new recovery system. As a raw material, NanSOt can be used. This salt is added to the recovery unit. Thus, Na2SO4 is reduced to NazCOs-l-NazS, which salts are converted by CO2 into NaHCOs and HzS. After decomposing NaHCOa to Na2CO3+COz, sodium carbonate is causticized with CaO to NaOH. Calcium carbonate, lime mud, which is formed in the causticizing operation is disposed of. With the CaCOa mud, impurities such as insoluble silica are removed from the .system which is a desired feature. CO2 removed in the causticizing is replaced with CO2 recovered from flue gases. Manufacturing of pure NaOH in this way has not been possible heretofore on an industrial scale for instance, in the sulfate industry, because the causticized liquor always contains sodium suliide. The present process is therefore novel and this feature is important.

(l0) It is very advantageous to use liquid SO2 in the improved cooking process. Any appropriate system of recovering and liquefying SO2 can be used. For example, the so-called Somer process described in the Norwegian Patent No. 73,832 or the usual procedure of compressing, cooling and liquefying. Thus, sulfur dioxide gases can be liquefied by conducting such gases into a vessel containing'aqueous solution of strong acid sodium bisuliite and by taking the strong gas (SO2) from the top of this vessel by means of a compressor and conducting it to the pipe for incoming gases going to a liquefaction plant. The gas on the top of the acid tank contains 8 to 10 kilograms per cubic meter of SO2 and therefore the cost of compressing is very low.

Although the present invention has been described in conjunction with certain preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such variations and modifications are to be considered within the purview of the application and the scope of the appended claims.

I claim:

l. The improved process of recovering chemicals and heat from residual liquor produced in a three-stage acidneutral-alkaline process for producing pulp from fibrous material containing cellulose which comprises cooking fibrous material with a weak acid solution containing an agent of the group consisting of sulfite, bisulte, and bisultite with an excess of S02 of an alkali metal, producing partly digested pulp, converting said acid cooking by injecting an alkaline liquor in the digestor at the end of the acid cooking stage to an alkali cooking liquor having a pH range of about 7 to about 10.5 Without separating said partly cooked pulp and without discontinuing the cooking, said injected alkaline liquor consisting essentially of a carbonate and mother liquor of an alkali metal produced in the conversion of the smelt solution produced hereinafter, cooking with said Weak alkaline cooking liquor consisting essentially of a carbonate of an alkali metal to produce completely digested pulp and residual liquor while evolving strong CO2 during said alkaline cooking, recovering said evolved CO2, said recovered CO2 constituting mainly the amount required in the subsequent treatment of a smelt solution, separating said pulp from said residual liquor, evaporating and burning said residual liquor to produce steam and a smelt consisting essentially of a carbonate of an alkali metal and a sulfide of an alkali metal, dissolving and clarifying said smelt solution, introducing gaseous CO2 provided mainly by the CO2 recovered in the aforesaid conversion of acid cooking liquor to alkaline cooking liquor into said smelt solution until substantially all of said sulde of the smelt solution is converted to a bicarbonate of an alkali metal and H28 gas, separating a portion of said bicarbonate from the smelt solution, using said portion to make acid cooking liquor used in the first step, using the remaining bicarbonate and mother liquor in subsequent steps, burning said HzS gas to SOz heat from residual liquor produced in a three-stage acidneutral-alkaline process for producing pulp from fibrous material containing cellulose which comprises cooking iibrous material with a weak acid solution containing an agent of the group consisting of sullite, bisulfite, and bisullite with an excess of SO2 of an alkali metal, converting the cooking liquor of the lirst stage by injecting an alkaline solution at the end of acid cooking stage to an alkali cooking liquor having a pH range of about 7 to about 10.5 without separating the pulp and without discontinuing the cooking, said injecting alkaline liquor consisting essentially of a bicarbonate, carbonate, and a sulfide of an alkali metal produced in the conversion of the smelt solution produced hereinafter, cooking with said alkaline cooking liquor consisting essentially of a carbonate and a sulfide of an alkali metal to produce completely digested pulp and residual liquor While evolving strong CO2 during said alkaline cooking, recovering said evolved CO2, said CO2 constituting mainly the amount required in the subsequent treatment of a smelt solution, separating said pulp from said residual liquor produced herein, evaporating and burning said residual liquor to produce steam and a smelt consisting essentially of a carbonate of an alkali metal and a sulde of an alkali metal, dissolving smelt in Water to form hot solution, adding to said hot solution an agent of the group consisting of calcium carbonate and calcium hydroxide to remove silicio acid and other impurities, clarifying said smelt solution, introducing gaseous CO2 provided mainly by the CO2 recovered in the aforesaid conversion of acid cooking liquor to alkaline cooking liquor into said smelt solution in such a manner that the sulfide of the smelt solution is incompletely carbonated While evolving HzS gas, crystallizing that amount of bicarbonate which is needed to make acid, separating said bicarbonate crystals from the smelt solution, and using the remaining smelt solution consisting of a bicarbonate, a carbonate, and a sulfide of an alkali metal to neutralize the acid cooking liquor of the lirst stage and to form the alkaline cooking liquor of the third stage, burning said HzS gas to SO2 gas, making cooking liquor used in the iirst step from said separated bicarbonate crystals and SO2 produced herein under pressure, to make weak acid cooking liquor containing an agent of the group consisting of a sulfite, bisullite, bisuli'ite with an excess of an alkali metal while evolving strong CO2 gas and recovering CO2 gas evolved herein for use in the step of treating the smelt solution to produce a bicarbonate of an alkali metal.

3. The improved process of recovering chemicals and heat from residual liquor produced in a three-stage acidneutral-alkaline process for producing pulp from brous material containing cellulose which comprises cooking fibrous material with a Weak acid liquor containing an agent of the group consisting of suliite, bisulte, and bisulfite with an excess of SO2 of an alkali metal to produce partly digested pulp, converting said acid cooking liquor at the end of the acid cooking stage to an alkaline cooking liquor having a pH range of about 7 to about 10.5 without separating the pulp and without discontinuing the cooking, said alkali cooking liquor consisting essentially of a crystallized carbonate of an alkali metal produced in the conversion of the smelt solution produced hereinfter, evolving strong CO2 from said liquor, recovering said evolved CO2, said CO2 constituting primarily the amount required in the subsequent treatment of a smelt solution, cooking with said weak alkaline cooking liquor consisting essentially of a carbonate of an alkali metal to produce completely digested pulp and residual liquor, evolving strong CO2 during said cooking, separating said pulp from said residual liquor, evaporating and burning said residual liquor to produce steam and a smelt consisting essentially of a carbonate of an alkali metal and a sulfide of an alkali metal, dissolving said smelt in water to form a hot solution, adding to said hot solution an agent of the group consisting of calcium carbonate and calcium hydroxide to remove silicio acid and other impurities, clarifying said smelt solution, cooling said smelt solution prior to treating same with an excess of CO2 under pressure provided primarily by the CO2 recovered from the aforesaid conversion of acid cooking liquor to alkaline cooking liquor, controlling the temperature of said smelt solution while treating with CO2 gas until substantially all of said sullide of an alkali metal is converted to bicarbonate and HgS gas, crystallizing said bicarbonate in said smelt solution, separating said smelt solution into mother liquor and said crystallized bicarbonate, using said separated crystallized bicarbonate in neutralizing the acid cooking liquor of the iirst stage and in forming the alkaline cooking liquor of the third stage, burning said HzS gas to SO2 gas, making weak acid cooking liquor used in the first stage from the mother liquor and from SO2 under pressure, said acid cooking liquor containing an agent of the group consisting of sulfite, bisulite, and bisulfite with an excess of SO2 of an alkali metal, evolving strong CO2 during the making of acid cooking liquor, and recovering strong CO2 gas evolved herein for use in the step of treating the smelt solution to produce a bicarbonate of an alkali metal.

4. The improved process of recovering chemicals and heat from residual liquor produced in a three-stage acidneutral-alkaline process for producing pulp from librous material containing cellulose which comprises cooking librous material with a Weak acid solution containing an agent of the group consisting of sullite, bisulite, and bisullite with an excess of SO2 of an alkali metal to produce partly cooked pulp, converting acid cooking liquor used in the first stage at the end of the acid cooking stage by injecting alkaline liquor to form an alkaline cooking liquor having a pH range of about 7 to about 10.5 Without separating said partly cooked pulp and Without discontinuing the cooking, said injecting alkaline liquor consisting essentially of decomposed bicarbonate and the mother liquor produced hereinafter in the conversion of the smelt solution produced herein, evolving strong CO2, recovering said evolved CO2, said CO2 constituting primarily the amount required in the subsequent treatment of a smelt solution, cooking with said Weak alkaline cooking liquor consisting essentially of a carbonate of an alkali metal to produce completely digested pulp and residual liquor, evolving strong CO2 during said alkaline cooking, separating said pulp from said residual liquor, evaporating and burning said residual liquor to produce steam and a smelt consisting essentially of a carbonate of an alkali metal and a sulfide of kan alkali metal, dissolving smelt in water to form a hot solution, adding to said hot solution an agent of the group consisting of calcium carbonate and calcium hydroxide to remove silicic acid and other impurities, clarifying said smelt solution, cooling said smelt solution prior to treating it with an excess of CO2 gas under pressure provided primarily by the CO2 recovered from the aforesaid conversion of acid cooking liquor to alkaline cooking liquor, controlling the temperature of said smelt solution during said treatment with CO2, continuing the treatment of said smelt solution with C02 gas under pressure While cooling the solution until substantially all said sulfide is converted to a bicarbonate and HzS, crystallizing said bicarbonate in said smelt solution, separating said smelt solution into mother liquor and said crystallized bicarbonate, using a portion of saidbicarbonate to make acid, decomposing the remaining bicarbonate crystals and mother liquor, producing a carbonate of an alkali metal and evolving strong CO2 gas, using said decomposed bicarbonate and said mother liquor in neutralizing the acid cooking liquor and in forming alkaline cooking liquor, burning said HzS gas to SO2 gas, making acid cooking liquor used in the first stage from said bicarbonate and SO2 produced herein by reacting a sulte of an alkali metal with a portion of SO2 until bisulte of an alkali metal is formed, by neutralizing said bisulte with a bicarbonate in a vessel under pressure, by

taking about one-half of said sulite produced herein with SO2 to form bisuliite and by reacting the remaining sulte with remaining SO2 thereby producing a weak acid cooking liquor containing an agent of the group consisting of sulte, bisullte, and bisultite with an excess of SO2 of an alkali metal, and recovering substantially all CO2 gas evolved herein for use in the step of treating the smelt solution to produce a bicarbonate of an alkali metal.

5. The improved process of recovering chemicals and heat from residual liquor produced in a three-stage acidneutral-alkaline process for producing pulp from iibrous material containing cellulose which comprises cooking ibrous material with an acid liquor containing an agent of the group consisting of sodium sulte, bisulfte, and bisulfite with an excess of SO2 to produce partly cooked pulp, converting the acid cooking liquor by injecting an alkali into the digester at the end of the acid cooking stage to an alkaline cooking liquor having a pH range of about 7 to about 10.5 Without separating the pulp and without discontinuing the cooking, said injecting alkaline liquor consisting of a carbonate and mother liquor produced hereinafter by the conversion of a smelt solution, evolving strong CO2 gas, recovering said evolved CO2 gas, said CO2 constituting primarily the amount required in the subsequent treatment of a smelt solution, cooking with said alkaline liquor to produce completely digested pulp and residual liquor, separating said pulp from the residual liquor, evaporating and burning said residual liquor to produce steam and a smelt containing mainly NazCOs and NazS, dissolving and clarifying said smelt solution, cooling said smelt solution prior to treatment with CO2, treating said smelt solution under pressure with an excess of CO2 gas provided primarily by the CO2 recovered from the aforesaid conversion of acid cooking liquor to alkaline cooking lliquor until substantially all of NazS is converted to NaHCOa and HzS gas While controlling the temperature of said smelt solution during said treatment to a range of about 30 C. to about 40 C., crystallizing said bicarbonate in said solution, separating said smelt into mother liquor and said crystallized NaHCOs, using a portion of said crystallized NaHCOs to make acid, using the remaining crystallized NaHCOz and the mother liquor in neutralizing the acid cooking liquor and in forming alkaline cooking liquor, burning said H28 gas to SO2 gas, making acid cooking liquor used in the rst stage from a portion of said bicarbonate and said SO2 under pressure, said cooking liquor containing an agent of the group consisting of sodium sulte and bisuliite, evolving strong CO2 gas, and recovering substantially all of CO2 gas evolved herein for use in the step of treating the smelt solution to produce a bicarbonate of an alkali metal.

6. The improved process of recovering chemicals and heat from residual liquor produced in a three-stage acidneutral-alkaline process for producing pulp from fibrous material containing cellulose which comprises cooking brous material with a weak acid solution containing an agent of the group consisting of sodium sullite, bisuliite, and bisulte with an excess of SO2 of an alkali metal to produce partly cooked pulp, converting the acid cooking liquor at the end of the first stage to an alkali cooking liquor having a pH range of about 7 to about 10.5, without separating said partly cooked pulp and without discontinuing the cooking by injecting in said cooking liquor alkali compounds consisting essentially of a carbonate of sodium and to minor extent compounds of sodium and sulfur including sodium thiosulfate and polysulfides,k evolving strong CO2 gas, recovering said evolved CO2 gas, said CO2 gas constituting primarily the amount required in the subsequent treatment of a smelt solution, cooking with said weak alkaline cooking liquor consisting essentially of a carbonate of sodium and compounds of sodium and sulfur derived from prior cooking operations and alkali injection thereby producing completely cooked pulp, evolving strong C02 during the alkaline cooking, separating said pulp from said residual liquor, evaporating and burning said residual liquor to produce steam and a smelt consisting essentially of a carbonate of sodium and a suliide of sodium, evaporating said residual liquor and burning the same to produce a smelt, dissolving said smelt in water to form a hot solution, adding to said hot solution an agent of the group consisting of calcium carbonate and calcium hydroxide to remove silicic acid and other impurities, clarifying said smelt solution, cooling said smelt solution prior to treatment with CO2, treating said smelt solution under pressure with an excess of CO2 gas provided primarily by the CO2 recovered from the aforesaid conversion of acid cooking liquor to alkaline cooking liquor until substantially all of Na2S is converted to NaHCOs and to H25 gas while controlling the temperature of said srnelt solution during said treatment to a range of about 30 C. to about 40 C., crystallizing bicarbonate of sodium in said solution, separating said smelt solution into mother liquor and crystallized NaHCOa, using a portion of said crystallized bicarbonate to make acid and the remaining bicarbonate crystals and mother liquor containing to a minor extent sodium thiosulfate, sodium polysultide and other compounds of sodium and sulfur in the conversion step and the formation of said alkaline cooking liquor, burning said HzS gas to SO2 gas, making the acid cooking liquor for use in the rst stage from said bicarbonate and SO2, the acid making involvingreacting NazSOs with a portion of SO2 produced until NaI-1803 is formed, neutralizing said NaI-i803 with said NaHCOs, under pressure to form NazSOs, taking about one-half of said NazSOs formed herein and reacting the same with SO2, reacting the remaining NazSOz With the remaining SO2 thereby producing a Weak acid salt solution containing an agent of the group consisting of NazSOs, NaHSOs, and NaI-1503 with an excess of SO2, recovering substantially all of the CO2 gas evolved herein, and using the same herein for the treatment of the smelt solution to produce NaHCOs.

7. The improved process of recovering chemicals and heat from residual liquor produced in a three-stage acidneutral-alkaline process for producing pulp from iibrous material containing cellulose which comprises cooking brous material with a Weak acid salt solution containing an agent of the group of sodium sulte, bisultite, and bisuliite with an excess of SO2, controlling the acid brous material ratio to a low ratio of the order of about 3:1, continuing the acid cooking to produce partly cooked pulp, converting said acid cooking by injecting an alkali liquor into the digester at the end of the acid cooking stage to an alkaline cooking liquor having a pH range of about 7 to 10.5 Without separating the cooked pulp and Without discontinuing the cooking, said injection alkali liquor consisting essentially of a carbonate and mother liquor of sodium produced by the conversion of a smelt solution hereinafter, evolving strong CO2, recovering said evolved COz, said CO2 constituting primarily the amount required in the subsequent treatment of a smelt solution, cooking with said weak alkaline cooking liquor consisting essentially of a carbonate of sodium to produce completely digested pulp and residual liquor, evolving strong C02 during said alkaline cooking, separating said pulp from said residual liquor, evaporating said residual liquor, recovering secondary condensate produced in said evaporation of said residual liquor for use in various steps of said three stages, burning said evaporated liquor to produce stearnand a smelt consisting essentially of Na2Co3 and Na2S, dissolving said smelt in Water to form a hot solution, adding to said hot solution an agent of the group consisting of CaCOs` and Ca(OH)2 to remove silicic acid and other impurities, clarifying said smelt solution, cooling said smelt solution prior to treatment with CO2, treating said smelt solution under pressure with an excess of CO2 gas provided primarily by the CO2 recovered from the aforesaid conversion of acid cooking liquor to alkaline cooking liquor until substantially all of NazS is converted to NaHCOs and to HgS gas while controlling the temperature of said smelt solution during said treatment, crystallizing bicarbonate in said solution, separating said smelt into mother liquor and said crystallized NaHCOa, using a portion of said NaHCOs to make acid and remaining NaHCOa crystals and mother liquor in the neutralizing of acid cooking liquor and the formation of said alkaline cooking liquor, burning said HzS gas to SO2 gas, producing acid cooking liquor for use in the first stage from said bicarbonate and SO2, the acid making involving reacting NazSOa with a portion of SO2 until NaHSO3 is formed, neutralizing said NaHSOs with said NaHCOa under pressure, taking about one-half of said Na2SO3 and reacting the same with SO2, reacting the remaining NazSOs with the remaining SO2, thereby producing an acid cooking liquor containing an agent of the group consisting of Na2SO3, NaHSOa, and NaHSOs with excess of SO2, and recovering substantially all of the CO2 gas evolved herein for use in the step of treating the smelt solution to produce NaI-ICOS.

8. The improved process of recovering chemicals and heat from residual liquor produced in a three-stage acidneutral-alkaline process for producing pulp from fibrous material containing cellulose which comprises cooking fibrous material in a digester with a weak acid solution containing an agent of the group consisting of NazSOs, NaHSOa, and NaHSOs with an excess of SO2, controlling the acid/fibrous material ratio to a low ratio of the order of about 3:1, bleeding some of the acid cooking liquor from the digester at the end of acid cooking stage for use in the various operations hereof, converting the acid cooking liquor by injecting an alkali liquor into the digester at the end of the acid cooking stage to an alkaline cooking liquor having a pH range of about 7 to about 10.5 without separating the pulp andwithout discontinuing the cooking, said injection alkali liquor consisting of a carbonate of sodium and mother liquor of sodium produced hereinafter in the conversion of a smelt solution, evolving strong CO2 gas, recovering said evolved CO2 gas, said CO2 gas constituting firstly the amount required in the subsequent treatment of a smelt solution, cooking with said alkaline liquor to produce completely digested pulp and residual liquor, separating said pulp from the residual liquor, evaporating and burning said residual liquor in a furnace to produce steam and a smelt containing mainly NanCOa of metal and NazS, dissolving said smelt to form a solution and clarifying said smelt solution, cooling said smelt solution prior to treatment with CO2, treating said smelt solution under pressure with an excess of CO2 gas provided firstly by the CO2 recovered from the conversion of acid cooking liquor to alkaline cooking liquor until substantially all of NazS is converted to NaHCOa and to HzS gas while controlling the temperature of said smelt solution during said treatment, crystallizng said bicarbonate in said solution, separating said salt into mother liquor and said crystallized NaHCO3, using a portion of said NaHCOs to make acid and the remaining NaHCOs crystals and acid mother liquor in neutralizing the acid cooking liquor and the formation of the alkaline cooking liquor, burning said HzS gas to SO2 gas producing acid cooking liquor used in the rst stage from said bicarbonate produced herein and SO2 produced herein, the acid making involving reacting Na2SO3 with a portion of SO2 until NaHSOs is formed, neutralizing said NaHSOa with said NaHCO3 under pressure, taking about one-half of said NazSOs and reacting the same with SO2 and reacting the remaining NazSOa with the remaining SO2, the cooking liquor so produced containing an agent of the group consisting of Na2SO3,NaHSO3 and NaHSOs with an excess of SO2, recovering CO2 evolved herein from the decomposition of NaHCOs and from auxiliary operations including the neutralization of the liquor bled from the digester and from the ue gases coming from said furnace, and liquefying a portion of the SO2 produced herein for use in the first stage of cooking in a weak acid solution.

9. The improved process of recovering chemicals from liquor produced in pulping processes using as the cooking agent a solution containing an agent of the group consisting of sulfite, bisulte, and bisulfite with an excess of SO2 of an alkali metal comprising converting cooking liquor after finishing acid cooking to an alkaline liquor by treatment with a carbonate of an alkali metal while evolving and recovering CO2 under pressure, said recovered CO2 constituting mainly the amount required in the subsequent treatment of a smelt solution, evaporating said alkaline liquor, burning said evaporated alkaline liquor under reducing conditions, producing a smeltl containing mainly sulde and carbonate of an alkali metal, dissolving said smelt to form a smelt solution, clarifying said smelt solution, cooling said smelt solution, treating said cooled smelt solution with gaseous CO2 under pressure provided mainly by the C02 recovered from the aforesaid conversion of acid cooking liquor to alkaline liquor thereby producing bicarbonate crystals of an alkali metal and HzS gas, separating a portion of bicarbonate crystals to make acid cooking liquor, using the remainder of said bicarbonate crystals and mother liquor for converting the aforesaid cooking liquor to said alkaline liquor, burning said H2S to SO2 gas, making acid cooking liquor from said separated bicarbonate crystals and from said SO2 gas while evolving strong CO2 gas under pressure, said acid cooking liquor containing an agent of the group consisting of sulte, bisulte, and bisulte with an excess of SO2 of an alkali metal, and recovering said evolved CO2 gas for re-use in treating smelt solution.

10. The improved process of recovering chemicals and heat from liquor produced in pulping processes using as the cooking agent a solution containing an agent of the group consisting of suliite, bisulfte, and bisulte with an excess of SO2 of an alkali metal comprising converting acid cooking liquor after finishing acid cooking to an alkaline liquor having a pH range of about 7 to about 10.5 by treatment with a carbonate of an alkali metal while evolving and recovering substantially pure CO2 under pressure, said recovered CO2 constituting mainly the amount required in the subsequent treatment of a smelt solution, evaporating said alkaline liquor, burning said evaporated alkaline liquor under reducing conditions to produce a smelt containing mainly sulfide and carbonate of an alkali metal while recovering heat, dissolving said smelt to form a smelt solution, clarifying said smelt solution, cooling said smelt solution, treating said cooled smelt solution with said recovered substantially pure gaseous CO2 under pressure provided mainly by the CO2 recovered from the conversion of acid cooking liquor to alkaline liquor thereby producing bicarbonate crystals of an alkali metal and HzS gas, separating a portion of bicarbonate crystals to make acid cooking liquor, using the remainder of said bicarbonate crystals and mother liquor for converting the aforesaid cooking liquor to said alkaline liquor, burning said H28 to SO2 gas, making acid cooking liquor from said separated bicarbonate crystals and from said SO2 gas under pressure while evolving substantially pure CO2 gas, said acid cooking liquor containing an agent of the group consisting of sulte, bisuliite, and bisuliite with an excess of SO2 of an alkali metal, and recovering said evolved, substantially pure CO2 gas for re-use in treating smelt solution.

l1. The improved process of recovering chemicals and heat from liquors produced in a pulping process which comprises cooking fibrous material in a digester with an acid solution containing an agent of the group consisting of NazSOa, NaHSOs, and NaHSO3 with an excess of SO2, converting the acid cooking liquor at the end of the acid cooking stage to an alkaline liquor having a pH range of about 7 to about 10.5 by adding an alkaline liquor, said added alkaline liquor consisting of a carbonate of 23 sodium and mother liquor produced hereinafter in the conversion of a smelt solution, evolving CO2 gas under pressure, recovering said evolved CO2 gas, said recovered CO2 gas constituting mainly the amount required in the subsequent treatment of a smelt solution, evaporating said alkaline liquor, burning said evaporated alkaline liquor under reducing conditions to produce a smelt containing mainly sulde and carbonate of sodium, dissolving said smelt in Water to form a smelt solution, clarifying said smelt solution, cooling said smelt solution containing sulfide and carbonate of sodium to a temper f about 30 C. to about 40 C., carbonating said cooled smelt solution with an excess of gaseous carbon dioxide under pressure provided mainly by the CO2 recovered in the aforesaid conversion of acid cooking liquor to alkaline liquor to produce a slurry containing crystals of sodium bicarbonate and to evolve HzS while continuing to cool the solution undergoing carbonation, filtering said slurry to remove bicarbonate crystals and to leave a solution containing bicarbonate of sodium as an alkaline mother liquor, using mother liquor containing a carbonate of sodium to convert the acid cooking liquor to alkaline liquor, adding water to the aforesaid bicarbonate crystals to form an aqueous mass consisting of a member of the group consisting of a slurry and a solution, decomposing said bicarbonate to evolve substantially pure car bon dioxide under pressure and to make a fresh solution containing a carbonate of sodium, burning the aforesaid HzS gas to form SO2 gas, treating a solution of NazSOs with SO2 gas to form a solution of NaHSOa While venting waste gases therefrom, neutralizing said solution containing sodium bisulte with said fresh carbonate solution to produce a suliite solution while evolving substantially pure carbon dioxide under pressure, dividing said solution containing sodium sulte into two portions, using one portion of said sulfite solution in the aforesaid step to form a bisulte solution whereby a gas-free bisulte solution is available for the aforesaid neutralization with the fresh carbonate solution, treating said second portion of sullite solution with SO2 gas to produce an acid cooking liquor containing sodium sulte, sodium bisulte, and sodium bisulfite and excess sulfur dioxide for cooking in the pulping process, and collecting the carbon dioxide evolved in the aforesaid operations under pressure for re-use in the treatment of the smelt solution.

References Cited in the tile of this patent UNITED STATES PATENTS 1,605,927 Drewsen Nov. 9, 1926 1,787,953 Richter Ian. 6, 1931 1,787,954 Richter Jan. 6, 1931 1,934,655 Bradley et al Nov. 7, 1933 1,983,789 Bradley et al. Dec. l1, 1934 2,192,239 Palmrose Mar. 5, 1940 2,221,066 Kahle Nov. 12, 1940 FOREIGN PATENTS 43,716 Norway Feb. 14, 1927 87,098 Sweden Aug. 1l, 1936 

1. THE IMPROVED PROCESS OF RECOVERING CHEMICALS ANDF HEAT FROM RESIDUAL LIQUOR PRODUCING IN A THREE-STAGE ACIDNEUTRAL-ALKALINE PROCESS FOR PRODUCING PULP FROM FIBROUS MATERIAL CONTAINING CELLULOSE WHICH COMPRISES COOKING FIBROUS MATERIAL WITH A WEAK ACID SOLUTION CONTAINING AN AGENT OF THE GROUP CONSISTING OF SULFITE, BISULFITE, AND BISULFITE WITH AN EXCESS OF SO2 OF AN LAKALI METAL, PRODUCING PARTLY DIGESTED PULP, CONVERTING SAID ACID COOKING BY INJECTING AN ALKALINE LIQUOR IN THE DIGESTOR AT THE END OF THE ACID COOKING STAGE TO AN ALKALI COOKING LIQUOR HAVING A PH RANGE OF ABOUT 7 TO ABOUT 10.5 WITHOUT SEPARATING SAID PARTLY COOKED PULP AND WITHOUT DISCONTINUING THE COOKING, SAID INJECTED ALKALINE LIQUOR CONSISTING ESSENTIALLY OF A CARBONATE AND MOTHER LIQUOR OF AN ALKALI METAL PRODUCED IN THE CONVERSION OF THE SMELT SOLUTION PRODUCED HEREINAFTER, COOKING WITH SAID WEAK ALKALINE COOKING LIQUOR CONSISTING ESSENTIALLY OF A CARBONATE OF AN ALKALI METAL TO PRODUCE COMPLETELY DIGESTED PULP AND RESIDUAL LIQUOR WHILE EVOLVING STRONG CO2 DURING SAID ALKALINE COOKING, RECOVERING SAID EVOLVED CO2, SAID RECOVERED CO2 CONSTITUTING MAINLY THE AMOUNT REQUIRED IN THE SUBSEQUENT TREATMENT OF A SMELT SOLUTION, SEPARATING SAID PULP FROM SAID RESIDUAL LIQUOR, EVAPORATING AND BURNING SAID RESIDUAL LIQUOR TO PRODUCE STEAM AND A SMELT CONSISTING ESSENTIALLY OF A CARBONATE OF AN ALKALI METAL AND A SULFIDE OF AN ALKALI METAL, DISSOLVING 